The present invention relates to the improvement of the so-called extrusion coating apparatus and more particularly to the extrusion coating apparatus wherein a wetting property between a web and coating liquid especially for the liquid with a high viscosity has been improved and thereby a uniform coated layer can be obtained.
A conventional extrusion coating machine and a backup roll is illustated in FIG. 1. In the figure, 1 is a web, 2 is the first block of the coating apparatus having on its tip an edge 3 stretching in the direction of the width of a web 1, and 4 is the second block formed in the same manner as the first block 2 and having an edge 3'. Between the first block 2 and the second block 4, the slit 5 is formed and the total amount of the coating liquid 6 is supplied from the slit 5 to the narrow gap h as the edge 3 runs parallel to the web 1. When such coating apparatus is used for coating, it is known that the coating is made with a relation of "h=2hw," where the thickness of a coated layer is hw, and it is also known that the relation is dependent on the physical properties (viscosity in particular) of coating liquid and a coating speed. However, when a uniformity of the coated layer is required on a high accuracy basis like the case of photographic photosensitive material or of magnetic recording material and others, there have been disadvantages that the position X where the coating liquid 6 first touches the web 1 (hereinafter referred to as "a coating liquid contact line") will be zigzagged in the direction of the width of web 1 when the viscosity of coating liquid 6 is high, and streak defects tend to be generated in the traveling direction of the web 1. This has been caused by insufficient wetting between coating liquid 6 and the web 1, in particular, so-called coating defects are generated in the form of cross lines wherein wetting portions and non-wetting portions appear alternately on the surface of the web 1 in its lateral direction. Further, there has been an inconvenience that the thickness of a layer is directly influenced by the variation in the gap h between the edge 3 and the web 1 caused by unavoidable causes such as an eccentricity of the backup roll (not illustrated), for example, or the thickness variation of the web 1, because the thickness of the coated layer is controlled by the relation of "hw=h/2" as mentioned before.
The inventors of the present invention, after an intensive study for the solution of aforesaid problems, found that the problem of coating defects mentioned above may be solved to a certain extent by an improvement in the wetting property of the web or the means therefore, such as lowering of the surface tension of the liquid or the like. It is further found that the problem of coating defects becomes pronounced especially when the viscosity is high and the coating speed is increased.
The position of the coating liquid contact line is controlled by the wetting property of the web surface and by the surface tension of the liquid and further by the pressure generated by the flow of coating liquid in the gap between the web and the edge. Such pressure depresses the contact line in the direction opposite to the traveling direction of the web against the force of the web to carry the liquid away in its traveling direction and thus the position of the contact line is settled by the balance of these forces. Therefore, as the viscosity of the liquid becomes higher or as the coating speed becomes higher, the force of the web to carry the liquid away becomes stronger and exceeds the pressure generated, and thereby the balance is lost causing a zigzag of the contact line in the direction of the width and the coating defects mentioned above thus to take place.
Further, as the second cause for coating defects, air tends to enter the gap between the second block of the coating apparatus and the web for the high speed coating whereby the streak defects are considered to take place. Such defects may be solved if the gap is eliminated, but without a gap the web tends to be damaged by the backup roll and the coating apparatus and if there should be dust or other items, the web will be torn. Therefore, the gap can not be eliminated.
After studying such causes, the inventors of the present invention found that all aforesaid problems will be solved by increasing the pressure to be generated by the flow of the coating liquid in the gap and further found that this will solve aforesaid problem of variation in the thickness of the coated layer.